The invention relates to a protective gas for annealing steel and other metals. For bright-annealing of steel and other metals, protective gases have been used for many years, which gases were obtained by partial combustion of gas or oil. The CO-, H.sub.2 -, CO.sub.2 -, CH.sub.4 - and N.sub.2 -contents of these gases differ very much, according to the starting fuel, the factor for the air of combustion, and the process used for the production of the protective gas. However, common to these gases are low O.sub.2 - and H.sub.2 O-contents.
For some time nitrogen, which is obtained by decomposition of air, has been used to an increasing extent for bright-annealing. Conditioned by the production process, the O.sub.2 - and H.sub.2 O-contents of the nitrogen areextremely low. It has been proved by numerous labor- and plant-bright-annealing tests, and its use for years in operation that nitrogen is suitable for bright-annealing of many steel and metal kinds, in different furnaces, and that the addition of reducing gases (e.g. H.sub.2) is not required. When in spite of this, numerous annealings add to the nitrogen 1-5% H.sub.2, then either in order to bind the oxygen penetrating through leakiness of the furnace or to be able to reduce oxygen compounds which are brought into the furnace with the annealing material and/or with the rolling- or drawing material adhering to it, are brought into the furnace.
The method of the supply of nitrogen to the annealings depends substantially upon the amounts required. For little consumption, gas bottles are used for medium consumption bales of bottles. If the consumption is higher, liquid nitrogen is supplied and is stored in the liquid, intensely-cold state in a container suitable for it. The amount required in each case is converted upon demand into the gaseous state and conducted to the place of consumption. In the case of bottle- and bales of bottle supply, N.sub.2 --H.sub.2 -mixtures are delivered in the desired concentration as so-called forming gases.
The hydrogen, which is required for the production of N.sub.2 --H.sub.2 -mixtures, when liquid nitrogen is used, is delivered in gaseous form in bottles or bales of bottles to the consumer and is mixed there with the nitrogen in the desired concentration, by means of a mixing device.
This method has been approved and found reliable for years. However, a disadvantage of it is that the costs for a mixing device must be spent, and that the transportation and handling of glass bottles and bales of glass bottles is relatively expensive.
These disadvantages are avoided by a process, which is based on the solubility of hydrogen in liquid nitrogen, in dependence upon the partial hydrogen pressure existing above the liquid phase. The advantages of this process are that no mixing device is required, and that -- inasmuch as the N.sub.2 --H.sub.2 -mixture is delivered in the liquid state -- the relatively expensive handling with H.sub.2 -bottles or H.sub.2 -bales of H.sub.2 -bottles, and the transportation of the glass bottles or bales of glass bottles is eliminated. A disadvantage of this process consists in the fact that hydrogen dissolves in liquid nitrogen only, when above the liquid phase a correspondingly high H.sub.2 -partial pressure (about 1 bar/0.2% H.sub.2) is maintained. It results therefrom that the cold gasifiers must be driven with high operating pressures, when higher H.sub.2 -contents (e.g. 3-5%) shall be adjusted. Another difficulty results from the fact that the H.sub.2 -partial pressure above the liquid phase must not be substantially changed, when defined H.sub.2 -contents shall be maintained. By each withdrawal from the gas phase above the liquid phase, the H.sub.2 -content of the mixture is reduced. Beyond that, there exists withdrawal from the high H.sub.2 -containing gas phase, danger of explosion, when as upon working with low-percentage N.sub.2 --H.sub.2 -mixtures no such customary precautionary means are taken. Furthermore, that the cold-gasifier and tank vehicle must be made H.sub.2 -tight, must be considered a disadvantage.